Growers who replace high-pressure sodium lamps with LED often report that the crop "looks colder" at the same air temperature. That impression is measurable, it has a physical cause, and it changes how the room should be run. This article separates the radiation physics from the physiology and gives a setpoint checklist for a retrofit.
A leaf sits in an energy balance. It absorbs radiation, exchanges sensible heat with the air through convection, and loses latent heat when water evaporates through open stomata. Leaf temperature is the value at which those flows balance, so it can sit above or below air temperature depending on which term dominates.
Under a lamp that radiates strongly in the thermal infrared, absorbed radiation pushes the balance upward and the leaf runs warmer than the air. Under a lamp that delivers mostly visible photons and rejects its waste heat to the room air by convection, the balance is set mainly by convection and transpiration, and the leaf can run slightly cooler than the air.
That is why a room can read a comfortable 22 °C on the sensor while the canopy behaves as if it were several degrees different. Irrigation demand, cell expansion, flowering timing and the risk of tipburn all respond to the leaf rather than to the sensor.
The difference is not a matter of total heat produced — both fixtures convert the same electrical input into roughly the same total heat at the same efficacy. It is a matter of where that heat goes.
An older but still useful framing: the room-level heat load may be almost identical, but the distribution of that load between the canopy and the air mass differs. That is the whole reason a like-for-like swap produces a crop that feels different without any change in the thermostat reading.
The most directly comparable published dataset for this question is a greenhouse experiment on young tomato plants grown under LED (95 % red, 5 % blue) or HPS supplemental light, in four trials that differed in the fraction of lamp light relative to daylight. The paper reports lower leaf temperature and higher gas exchange under LED.
| Parameter | HPS | LED | Reported difference |
|---|---|---|---|
| Leaf temperature | Higher | Lower | Direction consistent across trials; magnitude shrank as the lamp share of total light fell |
| Stomatal conductance (gs) | 0.32–0.59 mol H₂O m⁻² s⁻¹ | 0.35–0.66 mol H₂O m⁻² s⁻¹ | About 6 % higher under LED on average; 4–20 % by genotype |
| Transpiration rate (E) | Lower | Higher | 2–24 % higher under LED, depending on trial |
| Stomatal density | Lower | Higher | About 14 % higher under LED on average |
| Photosynthetic capacity | — | — | Young plants under LED tended to show higher capacity |
Two cautions belong with that table. First, the experiment used young tomato plants in a greenhouse with natural light; the effect scales with the share of light coming from lamps, and in a fully enclosed plant factory the lamp is essentially the whole environment, so the radiation term dominates even more. Second, the physiological responses were genotype-dependent — the paper reports that most genotypes gained 4–20 % in stomatal conductance under LED, while one genotype was 5 % lower. Do not treat a single number as universal.
It is tempting to conclude that a cooler canopy is simply healthier. The mechanism is more nuanced:
| Variable | What changes | First action |
|---|---|---|
| Air temperature setpoint | Canopy runs cooler relative to air | Measure leaf temperature before changing anything; raise air setpoint only if the crop is genuinely behind |
| Relative humidity / VPD | Boundary-layer humidity rises; stomata are more open | Increase air movement first, then adjust dehumidification; target VPD at the canopy, not at the sensor |
| Irrigation frequency | Transpiration typically rises 2–24 % at equal PPFD | Check substrate EC and moisture daily for the first two weeks; expect more frequent, smaller shots |
| PPFD / photoperiod | No change required for the comparison to hold | Keep PPFD constant while diagnosing, so only the spectrum and heat-delivery terms change |
| Leaf temperature | Now a diagnostic rather than a constant | Log it weekly with an infrared thermometer at the same canopy height and time of day |
| Heating pipe or rail temperature | Less radiant heat from above | Compensate with root-zone or pipe heating rather than by overheating the air |
You do not need a thermal camera to work with this, although one helps. A calibrated infrared thermometer with a known spot-size ratio, aimed at the same canopy layer at the same time each day, will show whether the crop sits above or below air temperature and how that gap changes after a retrofit.
Thermal imaging extends this to a spatial picture and is the basis of the crop water stress index used in the study cited above, but for routine greenhouse decisions the hand-held spot measurement is sufficient.
At the same PPFD and air temperature, the leaf-to-air relationship usually shifts downward under LED, because HPS delivers a large share of its energy as longwave infrared aimed at the canopy while LED rejects most waste heat to the air by convection. The size of the shift depends on how much of the total light comes from lamps.
Because the leaf, not the air, drives expansion, transpiration and development. A different radiation load changes leaf temperature even when the air sensor reads the same value.
Only if measurements show the canopy is running behind. Blindly raising air temperature can push VPD and respiration in the wrong direction; measure leaf-minus-air difference first, then adjust.
Usually not. In the published comparison, transpiration was 2–24 % higher under LED because stomatal conductance and stomatal density increased, outweighing the cooler leaf. Check substrate moisture rather than assuming.
It is stronger, because lamps supply essentially all the light. With no daylight to dilute the lamp spectrum, the radiation and convection terms are set almost entirely by the fixtures and the air handling.
Leaf temperature is the variable that connects a lighting change to a crop response, and it is the one most often left out of a retrofit plan. HPS and LED differ less in how much heat they release than in how they release it: radiation toward the canopy versus convection into the air. Measure the leaf-to-air gap before and after the change, then adjust humidity, air movement and irrigation to match what the canopy is actually experiencing.